问题
For variable assignment in Make, I see := and = operator. What's the difference between them?
回答1:
This is described in the GNU Make documentation, in the section titled 6.2 The Two Flavors of Variables .
In short, variables defined with := are expanded once, but variables defined with = are expanded whenever they are used.
回答2:
Simple assignment :=
A simple assignment expression is evaluated only once, at the very first occurrence.
For example, if CC :=${GCC} ${FLAGS} during the first encounter is evaluated to gcc -W then
each time ${CC} occurs it will be replaced with gcc -W.
Recursive assignment =
A Recursive assignment expression is evaluated everytime the variable is encountered
in the code. For example, a statement like CC = ${GCC} {FLAGS} will be evaluated only when
an action like ${CC} file.c is executed. However, if the variable GCC is reassigned i.e
GCC=c++ then the ${CC} will be converted to c++ -W after the reassignment.
Conditional assignment ?=
Conditional assignment assigns a value to a variable only if it does not have a value
Appending +=
Assume that CC = gcc then the appending operator is used like CC += -w
then CC now has the value gcc -W
For more check out these tutorials
回答3:
From http://www.gnu.org/software/make/manual/make.html#Flavors:
= defines a recursively-expanded variable. := defines a simply-expanded variable.
来源：https://stackoverflow.com/questions/4879592/whats-the-difference-between-and-in-makefile